Dead Zones to the Rescue

The following illustration is an artist’s conception of how gas and dust around a newborn star evolves into a planetary system. Later stage, the circumstellar disk is shown face-on, with planetary orbits as new ring structures.

Astronomers from McMaster University in Hamilton, Ontario find that "dead zones"- which typically extend out to 13 astronomical units from the central star of an extrasolar planetary system – can significantly slow planetary migration so that planets are not lost to the systems. This release summarizes work to be described in a press conference at the American Astronomical Society meeting in Calgary, Alberta on June 5, 2006, and in an invited talk at a scientific session during the meeting.

Surveys for extrasolar planets around thousands of stars like our Sun have now discovered that about 5 – 20 % of them harbour planets. More than 180 planets have been discovered, most have masses comparable to or greater than Jupiter, and all have orbits within 5 Astronomical Units (AU = distance between our Earth and the Sun) of their central stars. Some have orbits even closer to their stars than Mercury is to the Sun. It is widely believed that these planets formed far out (10 – 20 AU) in protoplanetary disks of gas and dust and then migrated to their present positions through the tidal interaction of the planet with the surrounding gaseous disk. This mechanism is highly efficient however, and standard models show that newly formed planets should have migrated and plunged into their central stars within a million years! How can planetary systems, that include low mass Earth-like planets as well as massive Jovian planets, be saved while still undergoing significant migration?

Ralph Pudritz, a Professor in the Department of Physics and Astronomy at McMaster University, will present an invited talk on "Planet Formation and Migration" at the AAS meeting, in which he will describe the results of a new theoretical discovery by Soko Matsumura, Ralph Pudritz, and Edward Thommes (paper submitted to the Astrophysical Journal), that shows that regions of very low gaseous viscosity known as "dead zones" – which typically extend out to 13 AU – can significantly slow planetary migration and save planetary systems. Low viscosity gas in a dead zone slows planet migration in two ways. The tidal force exerted by a massive planet on the disk will finally open a gap in the disk. The migration speed of the massive planet is then locked to the radial inward drift of gas (Type II migration) whose speed depends on how viscous the gas is.

Scientists can characterize a disk by looking at its infrared spectrum. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

The new theory shows that on entering a dead zone, such a planet opens a very wide gap, and migrates very slowly because of the very slow drift of gas in the dead zone. For low mass planets like the Earth, or even Neptune, migration in normal disks can occur quickly (within a million years) and such planets do not typically open gaps (Type I migration). The new calculations show that if a light planet starts from outside the dead zone, then its migration will be reversed on encountering a steep gradient in gas density that marks the outer edge of the dead zone. On the other hand, if light planet is formed within the dead zone in the first place, that it can open a gap in the dead zone and switch into the much slower Type II migration. As an example, the computer simulations show that a 10 Earth mass planet opens a gap at about 4 AU and slows its migration dramatically.

The models show that newly formed Earth-like planets will not splash into their central stars and often end up in orbits around 0.1 AU. Another major consequence of this work is that the prediction that most massive planets in other solar systems circle their stars in orbits beyond 5 AU, and are still waiting to be discovered.